1. Field of the Invention
The present invention generally relates to a self-healing coating using metallic microcapsules.
2. Description of the Related Art
The cost of corrosion is estimated to be at least $276 billion per year in the U.S. alone. A 2001 study commissioned by the Federal Highway Administration analyzed 26 industrial sectors to find that direct costs accounted for approximately 3.2% of the U.S. economy. Often overlooked in these numbers are the costs related to equipment downtime. For example, the time spent replacing or rehabilitating corroded equipment not only ties up valuable manpower, but also makes it necessary to maintain a reserve of excess capital equipment. With service rotations as short as 6 months, even a modest increase in service life can lead to significant savings.
The most common approach to preventing corrosion is to paint the surface with a protective coating. Typically, paints composed of an inorganic powder embedded within a polymer matrix have only limited ability to resist abrasion. Attempts to improve durability are ultimately constrained by the requirements that the coating be relatively thin (e.g., <100 μm) and easy to apply. While repainting and touch-ups can be performed as part of regular maintenance, many defects go unnoticed before significant damage occurs. Accordingly, self-healing coatings have been developed that autonomously repair scratches below some maximum width, thereby delaying the onset of corrosion and increasing the time between maintenance cycles.
The most common strategies utilized in developing self-healing polymer coatings are to supply energy to the system to form new bonds, or supply additional material to the damage zone. Supplying energy to the system could be as simple as heating a polymeric coating to achieve melt and reflow. Other examples include the use of heat to activate a reversible Diehls-Alder reaction, applying UV light to initiate the polymerization of pendant vinyl groups, and the use of hydrogen bonded polymers near their effective melting temperature. The advantage of energy activation is the potential for unlimited healing capacity. However, heating is logistically impractical for large objects, and UV activation may not provide complete healing if pigments in the coating interfere with light absorption.
Another approach achieves self-healing by supplying additional material to the damage zone. For example, one technique for delivering a reservoir of fresh material to a scratch include the use of embedded polymer microcapsules incorporated into paints and primers. The microcapsules release the self-healing compound or compounds, most commonly as liquids, when the coating system is damaged. However, appropriate materials should be used to fabricate the microcapsule and its contents, else it may “deploy” before the coating is applied or, upon application, spontaneously deploy improperly, i.e., without a physical compromise of the coating such as abrasion or nicking. Further, unless the microcapsule is compatible with both its contents (the encapsulated repair compound) and its surrounds (the solvent), the “application” life of the resultant mixed product may be less than desirable.
Accordingly, there is a continued need for improved self-healing coatings that can be made in a simple, cost efficient manner.